The present invention relates to a method of preparing fuel cladding having three-layers: an outer substrate, a zirconium barrier layer, and an inner liner. More specifically, the invention relates to methods of forming such three-layer cladding in which the zirconium barrier layer includes alloying elements to provide some measure of protection from accelerated corrosion.
Nuclear reactors have their fuel contained in sealed cladding for the isolation of the nuclear fuel from the moderator/coolant system. The term cladding, as used herein, refers to a zirconium based alloy tube. Often the cladding will be composed of various layers including a zirconium alloy substrate; and an unalloyed zirconium barrier.
The cladding--nominally in the order of 0.030 inches thick--is formed in the shape of a tube with the nuclear fuel contained typically in pellet form therein. These pellets are stacked in contact with one another for almost the entire length of each cladding tube, which cladding tube is in the order of 160 inches in length. Typically, the cladding tube is provided with springs for maintaining the axial position of the fuel pellets and so-called "getters" for absorbing: excess moisture. The internal portions of the fuel rod are pressurized with helium to help conduct the heat from the fuel material to the cladding.
Zirconium and its alloys, under normal circumstances, are excellent for nuclear fuel cladding since they have low neutron absorption cross sections and, at temperatures below about 350.degree. C., are strong, ductile, extremely stable and relatively nonreactive in the presence of demineralized water or steam. "Zircaloys" are a family of corrosion-resistant zirconium alloy cladding materials. They are composed of 98-99% by weight zirconium, with the balance being tin, iron, chromium, and nickel. "Zircaloy-2" and "Zircaloy-4" are two widely-used zirconium-based alloys for cladding. Zircaloy-2 has on a weight basis about 1.2 to 1.7 percent tin; 0.13-0.20 percent iron; 0.06-0.15 percent chromium and 0.05 to 0.08 percent nickel. Zircaloy-4 has essentially no nickel and about 0.2% iron but is otherwise substantially similar to Zircaloy-2.
Zircaloy cladding defects may occur due to various causes including debris induced fretting and pellet-cladding interaction. In the first of these, debris lodges next to the cladding and vibrates or frets against the cladding wall under the influence of the passing steam/water mixture. Such vibration continues until the cladding wall is penetrated. Pellet-cladding interaction is caused by the interactions between the nuclear fuel, the cladding, and the fission products produced during the nuclear reaction. It has been found that this undesirable effect is due to localized mechanical stresses on the fuel cladding resulting from differential expansion and friction between the fuel and the cladding in coincidence with corrosive fission product species causing stress corrosion cracking.
To combat defects due to pellet-cladding interaction, some cladding includes pure zirconium barrier layers metallurgically bonded to the inner surface of the tubing. The pioneering work on barrier layer cladding is described in U.S. Pat. Nos. 4,200,492 and 4,372,817 to Armijo and Coffin, U.S. Pat. No. 4,610,842 to Vannesjo, and U.S. Pat. No. 4,894,203 to Adamson, each of which is incorporated herein by reference for all purposes. Barrier layers have been found to effectively prevent damage to the cladding due to interaction with the pellet. However, if the cladding wall is compromised in some manner (e.g. perforated or split by debris fretting), and water enters the fuel rod interior, the protection afforded by the barrier layer can be reduced. This is because the steam produced by water within the fuel rod can very rapidly oxidize the barrier layer. Because of the speed at which this type of corrosion occurs, it is sometimes referred to as "accelerated" corrosion.
To protect the zirconium barrier from oxidation should a cladding breach occur, a three-layer structure may be employed. See e.g., U.S. patent application Ser. No. 08/091,672 entitled METHOD FOR MAKING FUEL CLADDING HAVING ZIRCONIUM BARRIER LAYERS AND INNER LINERS and U.S. patent application Ser. No. 08/092,188 entitled INNER LINERS FOR FUEL CLADDING HAVING ZIRCONIUM BARRIER LAYERS, both of which were filed on Jul. 14, 1993 and assigned to the assignee hereof. Both applications are incorporated herein by reference in their entirety for all purposes. In addition to the substrate and zirconium barrier, three-layer cladding includes a very thin corrosion resistant inner liner bonded to the fuel side of the barrier. Typically, the inner layer will be made from a Zircaloy or modified Zircaloy. If the cladding is breached and stem forms in the fuel rod interior, the inner liner will protect the barrier from rapid oxidation. Although this three layer design represents a significant advance, methods of forming three-layer cladding can be difficult to implement to perfection. Sometimes cracks or tears in the very thin inner liner occur during fabrication.
While the methods of preparing three layer cladding taught in U.S. patent application Ser. No. 08/091,672 provide cladding having substantial protection against damage from pellet-cladding interaction and accelerated corrosion, it is still desirable to develop other methods for preparing claddings having the same or improved properties.